The present invention relates generally to optical filters for use in microfilm-reader or the like with high-magnification lenses for correcting chromatic aberration, and more particularly to an optical filter which comprises a plate of colored glass having band-pass filter characteristics and an interference multilayer film formed over the glass plate.
Conventionally, silver interference filters are known for correcting the chromatic aberration of the high-magnification lenses. However, the silver interference filters have the drawback of being deficient in the amount of transmitted light. Further, it is known as another method for correcting chromatic aberration to construct a lens system by combining lenses having different dispersions from each other. However, this method requires many lenses and therefore has the drawback of being expensive.
To overcome these drawbacks, band-pass filters are presently commercially available which comprise the combination of two colored glass plates as shown in FIG. 7. The band-pass filter is composed of a first colored glass plate 01 (Y50, product number, manufactured by Hoya Corporation) provided with a first multilayer film 02 in the form of a thin optical and a second colored glass plate 03 (HA30, product number, manufactured by Hoya Corporation) provided with a second multilayer film 04 in the form of a thin optical film, the glass plates 01 and 03 being cemented together, so that the filter passes only the light of wavelengths of 500 to 600 nm as seen in FIG. 8. The two colored glass plates 01 and 03 are each about 2.5 mm in thickness, while the multilayer films 02 and 04 formed over these glass plates include about 30 layers in total. Accordingly, the filter has the drawback of having a large thickness (about 5 mm) and necessitating a large space for installation. Further as will be apparent from FIG. 7, the filter has another drawback in that it has a transmittance as low as up to about 60% and passes only a small amount of light. Moreover, the filter is made up of a large number of components and is not as low as is expected in cost. Thus, the filter still remains to be improved.